The present invention relates to a method and to an apparatus for coating board-shaped piece material, especially printed circuit boards, on both sides.
After they have been provided with conductors, printed circuit boards are coated with a protective layer. The protective layer is a preferably UV-hardenable photographically exposable plastics material, lacquer or the like. After drying of the protective layer, it is exposed in a suitable form and developed at the exposed sites. The conductors are then free at those sites and can be electrically contacted. In the case of printed circuit boards that are provided with conductors on both sides, the second surface of the boards is also coated and treated analogously to the first surface. The printed circuit boards are coated in a coating installation which comprises a series of processing stations through which the printed circuit boards to be coated pass in succession.
The previously cleaned printed circuit boards are transported to the entrance of a coating station in which one side of the printed circuit boards is provided with a protective layer. The coating station comprises a pouring table with a free-falling pouring curtain under which the printed circuit boards are transported with the surface to be coated uppermost. If desired or necessary, a preheating station is arranged upstream of the coating station. There the previously cleaned printed circuit boards are preheated in a stream of hot air before being coated. From the preheating station, which is generally constructed as a circulating oven, the printed circuit boards are transferred to the coating station. When the first side has been coated, the printed circuit boards are transported to a vapour-removal and drying station arranged following the coating station. In the vapour-removal and drying station, hot air is passed over the coated surface of the printed circuit boards. In that manner, solvent contained in the coating is evaporated (removed by a current of air) and suctioned off, and the coated surface is at least partially dried. In the case of the known coating installations for coating printed circuit boards on both sides, the printed circuit boards are also turned in the vapour-removal and drying station so that the coated surface of the printed circuit board is facing downwards when it reaches the exit of the station while the side of the printed circuit board that has not yet been coated faces upwards and can be coated. For that purpose, the printed circuit boards are again transported under a pouring curtain of a coating station, are coated and finally subjected to vapour-removal and drying in a vapour-removal and drying station. The printed circuit boards, which have now been coated and dried on both sides, are then transported to other processing stations.
A coating installation constructed for coating printed circuit boards on both sides is described, for example, in U.S. Pat. No. 4,926,789. In this known coating installation, a second coating station and a second vapour-removal and drying station are arranged following the first coating station and the first vapour-removal and drying station. The printed circuit boards turned in the fast vapour-removal and drying station are transported with the as yet uncoated surface uppermost under the free-falling pouring curtain of the second coating station. Then, in the second vapour-removal and drying station, they are transported through one or two series-arranged circulating vapour-removal driers. Hot air is again conveyed over the coated surfaces in order to evaporate (remove by a current of air) solvent contained in the coating, the solvent is removed by suction and the printed circuit boards are dried. All of the processing stations of that coating installation for coating printed circuit boards on both sides are arranged in a line one behind the other. Although the pre-drying station and the vapour-removal and drying stations are generally constructed as vertical circulating driers, the overall length of the installation is nevertheless considerable. Accordingly, only relatively long industrial sheds are suitable as installation sites.
U.S. Pat. No. 5, 113,785 and 5,113,701 describe coating installations that permit printed circuit boards to be coated on one or both sides, as desired. The described coating installations comprise only a single coating station and a single vapour-removal and drying station, which is preferably constructed as a vertical circulating drier. If the printed circuit boards are to be coated on both sides, then, after being turned, they are again transported under the pouring curtain of the first coating station and then subjected to vapour-removal and drying in the single vapour-removal and drying station.
In the case of the installation described in U.S. Pat. No. 5,113,785, the coating station and the vapour-removal and drying station are arranged side by side in the longitudinal direction of the installation. Arranged upstream and downstream of the coating station and the vapour-removal and drying station are transverse conveyors which transfer the printed circuit boards from the transport line with the coating station to the adjacent transport line with the vapour-removal and drying station. For example, lifting belts that are arranged transversely to the longitudinal direction of the coating installation are used as transverse conveyors. In a second variant, U.S. Pat. No. 5,113,785 proposes lifting and pivoting rakes as transverse conveyors, which rakes transfer the printed circuit boards from the transport line with the coating station into the adjacent transport line with the vapour-removal and drying station and vice versa. By arranging the coating station and the vapour-removal and drying station side by side instead of one behind the other, the overall length of the coating installation is markedly reduced. However, additional transverse conveyors are required to transfer the printed circuit boards from the first transport line to the second, adjacent transport line. Those transverse conveyors are relatively complicated in terms of construction and require an exact matching of their functioning to the operating rhythm of the vapour-removal and drying station, on the one hand, and to that of the coating station, on the other.
The coating installation described in U.S. Pat. No. 5,113,701 is constructed in a single line and requires no additional transverse conveyors. In that integrated installation, the vapour-removal and drying station is arranged substantially above the coating station. The printed circuit boards which have been coated on one or both sides are subjected to vapour-removal and dried while they are being transported in specially constructed holding devices above the coating station back to the entrance to the coating station or to the exit of the coating installation. Apart from the requirement for special holding means for the printed circuit boards, that coating installation has a relatively great overall height because the vapour-removal and drying station is arranged substantially above the coating station.
The problem of the present invention is accordingly to remedy the shortcomings of those coating installations of the prior art. In particular, the invention is to provide a method and an apparatus for coating board-shaped piece material, especially printed circuit boards, on both sides, which do not require additional transverse conveyors. The apparatus is to have as short an overall length as possible. In addition, its overall height is to be small and is to be determined essentially only by the overall height of the vertical continuous flow driers.